U.S. Sen. Lamar Alexander, R-Tennessee, has endorsed Marco Rubio for the Republican nomination for president, giving his Senate colleague from Florida a second major endorsement from a statewide Tennessee officeholder.

Alexander's endorsement comes three days after Tennessee Gov. Bill Haslam pledged his support for Rubio in a display of the state's growing Republican establishment backing of Rubio amid the surge of GOP frontrunner Donald Trump.

In a statement Sunday, Alexander, a third-term senator, singled out the next president's responsibility to appoint U.S. Supreme Court justices, and he warned of that task going to Democratic frontrunner Hillary Clinton if a Republican does not win in November.

"Marco Rubio is the conservative candidate who can inspire us, win the election, and lead our country," Alexander said. "The stakes are high. If our nominee does not win, Hillary Clinton’s justices will control the Supreme Court for 30 years and we’ll be stuck with Obamacare forever.

"I have watched Marco up close. I have seen him take the lead in passing new laws to impose tough new sanctions on Hezbollah terrorists, and to get rid of incompetent managers who weren’t doing their jobs to help veterans. He has proposed good new ideas for giving students more options for college. I am convinced he would be a strong and effective president of the United States."

Tennessee's Republican and Democratic primaries are set for Super Tuesday this week. The state has 58 Republican delegates up for grabs, third most of all Super Tuesday states. Early voting in Tennessee ended last week, with a record 254,659 Tennesseans voting early in the Republican primary.

Alexander and Haslam are scheduled to join Rubio for a campaign event in Knoxville on Monday morning that will showcase their endorsements.

Haslam, who is featured in a new Rubio television commercial, made his most forceful appeal yet for Rubio in a statement released ahead of Trump's campaign visit Saturday to Millington, Tenn., a small city outside of Memphis.

The second-term GOP governor Saturday called on Republicans to rally around Rubio to prevent Trump from taking over the Republican Party.

"It is time for Tennessee Republicans who do not want the party of Lincoln and Reagan taken over by Donald Trump to rally around Marco Rubio," Haslam said. "It is clear Marco is the only candidate who can beat Trump. With so much on the line for our country, we need Marco Rubio as our nominee because he is the conservative who can unite our party and win the general election."

Trump, a celebrity real estate mogul from New York, is on a roll after winning three consecutive states, and he's polling ahead in nearly ever Super Tuesday state.

Rubio drew more than 3,500 spectators at a campaign stop in Franklin last week, but a poll released last month showed him trailing Trump and U.S. Sen. Ted Cruz, R-Texas, by a wide margin in Tennessee.

With Alexander and Haslam's backing, Rubio now holds endorsements from two out of three of the state's top Republicans. Former U.S. Senate Majority Leader Bill Frist of Tennessee has also endorsed Rubio after previously being committed to Jeb Bush.

Meanwhile, U.S. Sen. Bob Corker, R-Tennessee, has indicated he won't endorse in the presidential primary.

Many Tennessee Republicans with close ties to Haslam and Alexander had supported Bush in the Republican primary, although neither of the two had formally backed the former Florida governor. When Bush dropped out of the race following his loss in South Carolina's primary, pressure started to build for the state's Republican establishment to get behind Rubio.

Tennessee’s last two Republican primary winners were former Arkansas Gov. Mike Huckabee in 2008 and U.S. Sen. Rick Santorum in 2012. Both benefited from strong support from evangelical Christians and weren’t the top picks of Tennessee Republicans in power.

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Republican presidential candidate and former Pennsylvania Sen. Rick Santorum, with his wife, Karen, left, speaks at his election night rally at Steubenville High School on March 6, 2012, in Steubenville, Ohio. He won the Republicans' Tennessee primary that night.
Eric Gay / AP

President Barack Obama waves before concluding a news conference in the James Brady Press Briefing Room of the White House on March 6, 2012. The president won the Democrats' Tennessee primary that night.
Pablo Martinez Monsivais / AP

Republican presidential hopeful and former Arkansas Gov. Mike Huckabee greets supporters at his election watch party as his wife, Janet, looks on in Little Rock, Ark., on Feb. 5, 2008. Tennessee was one of the primaries he won that night.
Danny Johnston / AP

Democratic presidential hopeful Sen. Hillary Clinton, D-N.Y., points to supporters at her Super Tuesday primary night rally in New York on Feb. 5, 2008. Tennessee was one of the states she won that night.
Elise Amendola / AP

President George W. Bush gestures to the audience Jan. 22, 2004, at the Roswell Convention and Civic Center in Roswell, N.M. He went on to win the Republicans' Tennessee primary on Feb. 10, 2004.
Lawrence Jackson / AP

Democratic presidential hopeful Sen. John Kerry, D-Mass., and his wife, Teresa, celebrate at a primary night party at George Mason University in Fairfax, Va., on Feb. 10, 2004. Tennessee was one of the states he won that night.
Charles Dharapak / AP

Republican presidential candidate Texas Gov. George W. Bush, center, is joined by his wife, Laura, right, and Sen. Kay Bailey Hutchison, R-Texas, left, during a victory celebration March 14, 2000, in Austin, Texas. Bush and Vice President Al Gore clinched their presidential nominations in a sweep of Southern primaries, including Tennessee, that night, securing the delegates they needed to ensure a November matchup.
David J. Phillip / AP

Democratic presidential candidate Vice President Al Gore waves with Sen. Bob Graham, D-Fla., during a campaign rally at Leon High School in Tallahassee, Fla., on March 14, 2000. Gore and Texas Gov. George W. Bush clinched their presidential nominations in a sweep of Southern primaries, including Tennessee, that night, securing the delegates they needed to ensure a November matchup.
Chris O'Meara / AP

President George H. W. Bush addresses the American Legislative Exchange Council on Feb. 21, 1992, in Washington, D.C. The president went on to win the Republicans' Tennessee primary March 10, 1992.
Barry Thumma / AP

Vice President George H. W. Bush holds up a Texas Victory shirt as his wife, Barbara, watches during his victory celebration on March 8, 1988, in Houston. Bush won big in the Super Tuesday primaries, including Tennessee's.
Ed Kolenovsky / AP

Sen. Albert Gore (D-Tenn). kicks off his bid for the Democratic presidential nomination with a telecast to 50 locations in Houston on Feb. 17, 1988. A couple of weeks later, on March 8, he won the Tennessee primary.
Ed Kolenovsky / AP

President Ronald Reagan holds up a University of Alaska-Fairbanks hockey sweatshirt that he was given during a rally on May 1, 1984, in Fairbanks at the university. He also won the Republicans' Tennessee primary that day.
Scott Stewart / AP

Presidential candidate Walter Mondale, center, gives the thumbs-up as three women eagerly await his handshake while Mondale was campaigning at a German restaurant and beer garden May 5, 1984, in Houston. He also won the Democrats' Tennessee primary that day.
R.J. Carson / AP

Republican presidential candidate Ronald Reagan and his wife, Nancy, meet with the press May 6, 1980, in Los Angeles. Reagan enjoyed substantial wins in the Republican primaries in Tennessee, Indiana and North Carolina and told reporters, "Tonight, I'm a little less cautious and a little more optimistic."
AP Photo / Nick Ut

President Jimmy Carter addresses the 1980 National Convention of the League of Women Voters at a Washington hotel on May 5, 1980. Carter, using the occasion to comment on the Cuban refugee situation, said he would end his self-imposed isolation and begin campaigning. He went on to win the Democrats' Tennessee primary the next day.
AP

President Gerald Ford reaches for the clapper of a bell that fell in his hand after ringing the bell during a ceremony commemorating Walnut Creek, Calif., war dead on his daylong campaigning in Northern California on May 25, 1976. He also won the Republicans' Tennessee primary that day.
AP

Democratic presidential hopeful Jimmy Carter watches a television monitor carrying vote totals May 25, 1976, during a visit to the CBS studios in New York. The news was good for the former Georgia governor, who won big in the Kentucky, Tennessee and Arkansas primaries.
Suzanne Vlamis / AP

President Richard Nixon pauses at the casket of J. Edgar Hoover after eulogizing the FBI director during funeral services at the National Presbyterian Church in Washington on May 4, 1972. He won the Republicans' Tennessee primary that day.
AP

Alabama Gov. George C. Wallace reads from his bed as he recovers from his shooting at Holy Cross Hospital in Silver Springs, Md., on May 17, 1972, with his wife, Cornelia. Wallace won the Democrats' Tennessee primary May 4, 1972.
AP